When examining an exoplanet's atmosphere is the star's emission spectra or planet's light used?

Question

My understanding of the main method we use to figure out an exoplanet atmosphere composition is that when a exoplanet transits their sun, visible light passes through the planet's atmosphere, and absorbed by the elements in the atmosphere. We then observe these absorption lines to figure out the chemical composition right? Or am I getting some details wrong here?

My main question is, does the light that passes through the planet's atmosphere is the sun's light or the sun's emission spectra?

Also will different stars starlight cause different absorption lines? As in say some planet X orbiting a M class star as opposed to the same planet X orbiting a G class star. Considering both the stars are different but the planet are same, would the absorption bands of the planet be different in each case?

References

• ESA: Starlight yields clues to exoplanets’ atmospheres
• Lecture by Michael Richmond: Spectroscopy of exoplanets/spectra/spectra.html

Answer 1

The three common techniques used for aquiring the spectra of exoplanets and their atmosphereseres are:

• Transmission: The brightness of a star decreases as the subject planet moves in front of it. If the planet has an atmosphere, it absorbs the suns emitted light. By measuring the brightness decrease at different wavelengths, the wavelength dependent transmittance of the atmosphere can be obtained. Here is a great example of how the chemical composition of the atmosphere is obtained from this measurments: Spectrally resolved detection of sodium in the atmosphere of HD 189733b with the HARPS spectrograph
• Reflectance: The star's light bounces off the planets atmosphere and gets reflected towards Earth. This can happen at allmost all points during the planets orbit, but ocurs specially when close to a secondaty eclipse (stars ocultation). Of course the measured light reflected from the planet is almost in its entirety the stars light, however, the host and the sun's emissions are separated by a fraction of an arcsecond. A good example of this method beign used: Searching for reflected light from τ Bootis b with high-resolution ground-based spectroscopy: Approaching the $10^{-5}$ contrast barrier
• Thermal emission: If the planet is hot enough, one can measure the radiation emission as if it were a blackbody. This can only be used (as of today) on big hot planets. Planets such as Earth are not measurable. A good example of this method: Atmospheric Circulation of Eccentric Hot Jupiter HAT-P-2b

Considering both the stars are different but the planet are same, would the absorption bands of the planet be different in each case?

It will not make any difference as the atmosphere's absorption lines are solely planet dependent and star's polychromatic light ensures same lines will show no matter what sun it is.

Answer 2

If you look for a simple answer by easy principles and not technicalities this would be the picture...

When the exoplanet transit between our point of sight and its star, its disk block part of the light. This results in a dimming of the star light we receive. The latter can be analyse as we do for light of any source. It is well approximated by a black body spectrum with dimmer lines corresponding to the absorption lines of the elements/species present in the star atmosphere.

If the exoplanetary disk that block the light has an atmosphere as well, the constituents of the latter will absorb their typical lines and this results on the appearance of dimmer and/or new lines in the star spectrum as received.

In principle there could be a scattering effect of the exoplanetary atmosphere, too. For instance an observer on the moon during a sun eclipse would observe the sun fainting overall, the absorption lines due to terrestrial air, and a global different spectral distribution toward the red due to scattering, also due to air.

However, in case of far exoplanetary systems, it is already astonishing that we can do the line analysis as for all the above just happen to the little portion of the star flux that is intercepted by the planet and at same time by our telescope.

My main question is, does the light that passes through the planet's atmosphere is the sun's light or the sun's emission spectra?

Emission by the star atmosphere to us or to the exoplanet is mixed with the proper photosphere emission. Practically, it is the star light whatever you like to call it. It has not really an influence on the difficult above measurement. They are difficult right because we must see tiny changes relatively to a much brighter and bigger (angular) sized source.

Considering both the stars are different but the planet are same, would the absorption bands of the planet be different in each case?

No. In principle there could be a difference in the easy of detection, but the absorption lines are typical of the absorber not of the excitation source. Obviously we can't search for absorption happening in the UV using just visible light. But the very broad emission of stars provides quite the same spectral coverage in term of wavelengths.